It is known that three-pole plugs and associated sockets are safe in use and therefore, they are now widely used in the connection of various kinds of electric appliances. FIG. 6 illustrates one of the commonly adopted three-pole plugs of which the inner structure can be clearly seen from the drawing. Such a conventional three-pole plug usually comprises an inner pin base member 6 and an outer protective housing member 7 fitly covering the inner pin base member 6. The inner pin base member 6 has a concave top 61 from an adequate point of there a cotter 62 suitable for engaging with an opening 31 at the end of a round-sectional ground terminal 3 upwardly projects. The inner pin base member 6 further has two side fixing blocks 63 suitable for separately engaging with end recesses 41, 51 of a positive terminal 4 and a negative terminal 5, respectively. The outer protective housing member 7 has two longitudinally extended and upward projected flanges 71 which are suitable for engaging with two corresponding grooves 64 formed at a bottom side of the inner pin base member 6. With the assembly of the outer protective housing member 7 and the inner pin base member 6 by engaging the flanges 71 with the grooves 64, the pre-assembled ground, positive, and negative terminals 3, 4, 5 are firmly fixed in place within the protective housing member 7. The assembled inner pin base member 6 and the outer protective housing member 7 must be wrapped with insulating material through injection molding so as to form the common three-pole plug as shown in FIG. 5.
In a further study of the above-mentioned manufacturing process and the structure of a conventional three-pole plug, it is found that the ground, the positive, and the negative terminals 3, 4, 5 are manually mounted on the inner pin base member 6, and then, the outer protective housing member 7 is manually put over to house the assembled terminals and inner pin base member 6. For an experienced and skilled operator, it might not be so difficult to complete this two-stage assembly, except a longer time is required. However, for an unexperienced operator, such assembly would be not only a time-consuming but also a labor-consuming work. Moreover, the assembled inner pin base member 6 must be housed with a protective housing member 7 which apparently increases the production cost due to additional material used to make the protective housing member 7.